Since a proton conductor conducts only proton, and has electrical properties as an insulator, it has been used as an electrolyte of fuel cells. Among such proton conductors, solid electrolytes composed of a solid polymer (for example, trade name Nafion) or a perovskite type solid oxide have been known, and used for stationary fuel cells, compact and portable fuel cells.
Performance of a proton conductor is evaluated with proton conductivity (Siemens per centimeter: S/cm). The proton conductivity represents the number of conducted protons per unit volume and unit time, and the proton conductivity in the temperature range employed serves as a basis for determining whether or not the proton conductor achieves favorable performance.
Proton conductors composed of a solid polymer currently put into practical applications conduct proton by means of an oxonium ion (H3O+) in the solid polymer. Thus, since the proton conductivity is exhibited in the state in which water is contained in a large amount in the solid polymer, this solid polymer is used as a solid electrolyte at a temperature not higher than 100° C. at which water evaporation is avoided.
On the other hand, since the proton conductor composed of a perovskite type solid oxide conducts proton by hopping of proton on oxygen constituting the solid oxide, it exhibits proton conductivity at a high temperature of not lower than 600° C. Accordingly, this solid oxide has been used as a solid electrolyte at a temperature of not lower than 600° C. In the proton conductor made of the solid oxide, the proton conductivity increases by setting the temperature in use to a higher temperature, whereas the proton conductivity decreases abruptly when used at lower temperatures.
When the operation temperature of a fuel cell is elevated, reaction efficiency of the catalyst is enhanced, leading to enhancement of efficiency of electric power generation; therefore, a proton conductor which can be used at higher temperatures has been desired. However, polymer solid electrolytes cannot be used at a temperature not lower than 100° C., as described above. On the other hand, there are many restrictions for reliability or durability of a fuel cell system for the operation of the fuel cell at a high temperature of not lower than 600° C. Thus, realization of a proton conductor which can be used in a temperature range of about 100° C. to 400° C. has been desired.
Under such circumstances, investigations of solid electrolytes that exhibit favorable proton conductivity in an intermediate temperature range of not less than 100° C. and not more than 600° C. have been carried out (see, for example, Patent Document 1).
Patent Document 1 discloses that tin pyrophosphate SnP2O2 is produced by adding phosphoric acid H2PO4 to tin oxide SnO2 followed by heating, and that thus obtained tin pyrophosphate exhibits high proton conductivity.